The present invention relates to a new process for the standardization and stabilization of organic polyisocyanates, to the polyisocyanates obtained by this process and to the use of those stable polyisocyanates to produce polyisocyanate addition products.
The nature and amount of impurities in polyisocyanates are determined by the method by which they are produced. The impurities cause fluctuations in the activity of the polyisocyanate and thereby affect the products made with the polyisocyanate. Reproducible and commercial application of impurity-containing polyisocyanates is therefore difficult.
In particular, the known phosgenation products of aniline-formaldehyde condensates (crude polyisocyanate mixtures of the diphenylmethane series) contain an abundance of such impurities. According to Chem. Soc. Rev. 3 (1974), p. 209 ff., chlorine-containing impurities which always cause activity fluctuations when "labile" (so-called hydrolyzable chlorine (HC value)) are of particular concern. Narrowing the range of fluctuation by reducing the amount of these impurities to standardize and improve activity is therefore of both technical and economic importance.
DD 285,593 teaches that organic polyisocyanates are purified at 200.degree. to 240.degree. C. by treating with compounds containing acetamide groups or .epsilon.-caprolactam and by subsequently blowing inert gases through the polyisocyanate.
GB Pat. 1,080,717 teaches that it is possible to reduce the hydrolyzable chlorine value (HC value) by thermal treatment at 180.degree. to 220.degree. C.
In addition to the large expenditure of energy, such high-temperature processes are dangerous due to the extraordinary reactivity of polyisocyanates which can oligomerize with a sudden evolution of heat.
DE-OS 2,614,323 discloses a process for the conversion of polyisocyanates in which a 1,2-epoxide is used as an acid acceptor. However, this process requires use of deactivating agents such as hydrogen halides, halides of phosphorus or tin or oxyhalides of phosphorus or sulfur. The mixture of the resulting modified polyisocyanates then contains fluctuating amounts of hydrolyzable chlorine.
The known processes disclosed in EP 445,608 and EP 445,602 for the standardization of polyisocyanates are unsafe, technically demanding or produce new defects as a result of additives.